[Michimura, Akutsu]
We have salveged glass flagments of the broken viewport window from the EXA room (the incident happend on 14th Mar 2016 in JST [kagra-pms 02950], although no reports are found in k-log). At the same time, we did some field investigations around the EXA chamber. Actually, this needed to be done immediately after the incident by suitable people with many evidences (concretely putting, pictures; pictures are must!!), and the field needed to be preserved until such activities were finished.
Though most of these had not been done, I slightly expected we can still learn something from the field even today.
A scenario proposed by Saftety Control Board of KAGRA [kagra-pms 02957] is that the window did not explode but was pop out from the flange pushed by an inverse pressure, and dropped to the floor, then broke to flagments; note that this incident happened during venting the chamber. The evidence of this hypothesis is that no glass flagments remains at the viewport flange.
Today, we found some tiny glass flagments were still exsiting in the 1-cm interspace between the optical table of the oplev QPD and the top of the pillar. If the window was just puped out, such glass flagments should not be here. At the same time, as reported here, glass flagments were also found on the QPD. So, I still suspect a certain level of an explosion happened.
One thing I'm worried about is on a camera attachement, which is fastened to the viewport flange with screws that also fastens the viewport flange to chamber flange. It seems the camera attachment is hitting the spring structure supporting the window in the viewport, and applying pressures. If this would be the cause, all such camera attachments need to be detached, and such viewports need to be replaced. Anyway, the spring structure of the broken viewport flange doesn't seem broken, apart from some scars probably generated by the camera attachment.
The camera attachment turned out to mechanically interfere with a large flange on the side of the EXA chamber, so the attachment was once detached. During this work, only 4 screws at the bottom of the flange are removed and then, the camera attachment was removed, then again the screws were tighten to fix the viewport to the chamber. So, one of the possible causes would be this non-unifrom torque.
Note that the viewport tolerate at least 2 evacuation.
Apart from these, we still find some glass flagments around the EXA chamber. We didn't move them, and only took pictures. Please be careful.
